The present invention relates to angular velocity sensors employed for the attitude control and navigation of moving objects such as an airplane, an automobile, robot, a ship, and other vehicles; for prevention of the still and video camera shake; and for the remote control for remotely operated equipment.
This type of angular velocity sensor is provided with a drive electrode and detection electrode on an U-shaped piezoelectric element. A tuning fork arm of the piezoelectric element is driven by the signal supplied from a single drive power connected to the drive electrode, and the angular velocity signal is taken from the detection electrode while the tuning fork arm is driven.
Recently, an angular velocity sensor using a single crystalline piezoelectric element typically made of quartz or lithium tantalate as an vibrating material has been proposed. This type of sensor is smaller, and has the possibility to supply more inexpensive angular velocity sensors than the type having a structure to attach a ceramic piezoelectric element onto a metal vibrator.
A conventional angular velocity sensor using a single crystalline pieozoelectric element has a pair of arms joined and fixed at their individual ends by a base to form a tuning fork vibrator. This tuning fork vibrator has, for example, an integral structure cut out from a quartz plate. A pair of drive electrodes are provided on one of the arms of the tuning fork vibrator as configured above for driving the tuning fork vibrator piezoelectrically in the direction parallel to a principal plane at a resonant frequency. These drive electrodes are electrically driven by an external oscillator circuit. A monitor electrode, sense electrode and ground electrode are provided on the other arm. The monitor electrode is for detecting the vibration amplitude caused by the oscillator circuit of the tuning fork vibrator. The sense electrode is for piezoelectrically detecting the stress due to the Coriolis force acting perpendicular to the principal plane of the arm against the angular velocity input along the axis direction of the tuning fork vibrator.
Herein, the electric charge generated in the monitor electrode is amplified by an external circuit and then compared with a reference signal preset by an auto gain control (AGC), to control the oscillator circuit for maintaining the vibration amplitude of the tuning fork vibrator constant. On the other hand, the detection electrode detects a signal due to the Coriolis force. This detected signal is amplified by an external amplifier circuit, and then synchronous detection is executed using the signal detected by the monitor electrode. The signal due to the Coriolis force modulated by the tuning fork vibrator is thereafter demodulated and the undesired frequency band is filtered out by an low pass filter (LPF) to generate the sensor output.
However, in the angular velocity sensor as described above, the drive signal induces and mixes a coupled capacity component to the detection electrode. This causes the need for providing another circuit to separate this mixed signal, but mixed signal is not completely separable. Remaining mixed signal becomes unwanted signal noise, degrading the detection characteristics of the sensor. This disadvantage has resulted in an inability to commercialize the conventional sensor on full-scale.
The present invention solves the above disadvantage, and aims to offer an angular velocity sensor with better detection characteristics by eliminating an effect of the noise generated by unwanted coupled capacity component.
In order to solve the above disadvantage, the angular velocity sensor of the present invention includes a tuning fork vibrator, and first, second, third, fourth, fifth, sixth, seventh, and eighth electrodes. The tuning fork vibrator includes a first vibrator which has at least two arms made of a single crystalline piezoelectric material and at least one base for connecting these arms, and a second vibrator which has approximately the same shape as that of the first vibrator and made of a single crystalline piezoelectric material. The first vibrator and the second vibrator have a crystal axis in which a piezoelectric phenomenon having inverse polarities in their width directions, and are directly bonded in the thickness direction for an integral structure to form the tuning fork vibrator which has at least two tuning fork arms and at least one tuning fork base. The first and second electrodes are disposed on the surface of both tuning fork arms of the tuning fork vibrator. The third and fourth electrodes are disposed on a rear face of the tuning fork arm at positions opposing the first and second electrodes respectively. The fifth, sixth, seventh, and eighth electrodes are disposed on a side face of both tuning fork arms of the tuning fork vibrator as detection electrodes. When the first electrode disposed on the surface of one of the tuning fork arms of the tuning fork vibrator or the third electrode disposed on the rear face of one of the tuning fork arms acts as a drive electrode, the drive power is connected in the next ways. If the first electrode is the drive electrode, the drive electrode in a phase different from that of the first electrode is supplied to the second electrode disposed on the surface of the other tuning fork arm. Alternatively, the drive signal in the same phase as that of the first electrode is supplied to the fourth electrode disposed on the rear face of the other tuning fork arm. If the third electrode is the drive electrode, the drive signal in the same phase as that of the third electrode is supplied to the second electrode disposed on the surface of the other tuning fork arm. Alternatively, the drive signal in a phase different from that of the third electrode is supplied to the fourth electrode disposed on the rear face of the other tuning fork arm. The fifth and eighth electrodes disposed on the outer face of both tuning fork arms are commonly connected, and the sixth and seventh electrodes disposed on the inner face of both tuning fork arms are commonly connected so that the detection signal may be taken at each of these commonly connected sections.
In this configuration, equivalent positive and negative unwanted coupled capacity components from the drive electrode is respectively and equally input to the detection electrodes. Or, unwanted coupled capacity component in the same polarity is equally and respectively input. This enables the cancellation of unwanted coupled capacity component in a respective set of detection electrodes commonly connected. Alternatively, unwanted coupled capacity component may be cancelled when the detected signals are differential-amplified. The effect of noise caused by the coupled capacity component can thus be eliminated, improving detection characteristics.